(CNN) -- A 10-year study examining 4.9 million births in the 1990s has found more evidence that there's a link between autism and the mother's age at conception.

"The risk of having a child with full syndrome autism increases with maternal age," concluded researchers at the University of California, Davis, who examined data from all births in their state for the decade. The findings are published in the February issue of the journal Autism Research.

The link between the parents' age and children's health is not entirely new. Prior studies have indicated that babies born to older women have higher risks of birth defects, low birth weight and certain chromosome problems, such as Down syndrome.

A 2007 Kaiser Permanente study conducted in California reported that autism risk increased with both the mother's and father's age. An Israeli study based in statistics from 1980s had isolated only paternal age as being linked with increased risk for autism.

Dr. Max Wiznitzer, a pediatric neurologist at Rainbow Babies & Children's Hospital in Cleveland, Ohio, said the latest research had a far larger sample size.

Autism is a growing disorder; the Centers for Disease Control and Prevention estimated that one in 110 children had the condition in 2006. But its causes remain unknown.

Video: Mother's age tied to autism

In the latest study, researchers found that mothers over the age of 40 had 51 percent higher odds of having children with autism compared with mothers between the ages 25 and 29.

The father's age also played a factor, but only when he had a child with a woman under 30.

"When the mom has minimal age risk of an autistic child, we do see increased risks as dads get older," said lead author Janie Shelton, a graduate student researcher at UC-Davis.

It's unclear why the mother's age has more bearing in autism risk than the father's.

The study authors emphasize that while autism rates have risen 600 percent in the past two decades, older women having children contributed to only 5 percent more cases of autism.

As more women delay childbearing, it's important to keep the study in perspective, said Geraldine Dawson, chief science officer of Autism Speaks, the nation's largest autism science and advocacy organization.

"When we look at that dramatic increase [of autism] over the last two decades, there are multiple factors that have contributed to this," she said. "It appears that advanced parents' age, not just mothers but also father's, account for a very small portion of that increase."

Shelton said older mothers should not jump to conclusions.

"I don't think a mom blaming herself is going to help us understand what's causing autism or help prevent further cases," she said. "I would urge parents not to blame themselves, regardless of what age they are."

Shelton and the co-authors obtained all birth records in California from 1990 to 1999 and then collected data from the state's Department of Developmental Services to count the number of autism diagnoses from children born during that decade.

How parental age increases autism risks remains unknown, but several hypotheses exist. Some suggest that the cumulative effects of the environment, changes to the autoimmune system, stress and reproductive technology may affect autism risk.

"As people age, we know there are changes to our DNA that occur," Dawson said. "There have been studies that show we have increased damage to our DNA as parents age. They're more likely to have children of low birth rate and more birth complications. It's not surprising that those factors would slightly increase the risk for autism as well as other neurological disorders."

Despite the lack of concrete answers, Shelton said, the findings offer some hints.

"It gives us some clues where to look biologically," Shelton said. "In an epidemiological study, age is a proxy for a lot of things. And so we're trying to further understand why age might be showing up as a risk factor, because we don't know the mechanism yet."